Notify of issuance data encoding costs / allow dust level tweaks

[jdogresorg]

Currently when a user enters a long description, the data encoding switches to multisig, which requires additional BTC in dust outputs to encode the asset description data.

Need to add a notification / confirmation window to let user know if the additional data encoding costs

[jdogresorg]

also should add option to freewallet to allow users to specify their own dust level (default to 1000 for multisig) in system settings.... so users can tweak their dust limits as needed

[jdogresorg]

update should be in next release